Increases in technological development and demand for mobile devices have resulted in a sharp increase in the demand for secondary batteries as an energy source. Among such secondary batteries, lithium secondary batteries, which have a high energy density and voltage, a long cycle life, and a low self-discharge rate, have been commercialized and are in wide use. However, such lithium secondary batteries have the limitation wherein repeated charging and discharging leads to a rapid decrease in lifetime. In particular, such limitations are more severe at high temperatures. The reason for this is that moisture inside the battery or other effects cause electrolytes to dissociate or active materials to degrade, and such is due to a phenomenon that occurs as a result of increased internal resistance in the battery.
Therefore, a positive electrode active material for lithium secondary batteries currently under active development is LiCoO2 having a layered structure. LiCoO2 is the most widely used due to having excellent lifetime properties and charge-discharge efficiency, but has low structural stability and thus has limits with respect to application in techniques for increasing battery capacity.
Various lithium transition metal oxides have been developed as positive electrode active materials for replacing LiCoO2, such as LiMnO2 and Li2MnO3 having layered structures, LiMn2O4, LiNiO2, and LiFePO4 having spinel structures, or Li(NixCoyMnz)O2 and the like.
Among these, lithium manganese oxides such as LiMnO2, Li2MnO3, and LiMn2O4 have the advantages of excellent thermal stability and low cost, but have the limitations of low capacity, and poor high-temperature properties.
Therefore, research on nickel-based positive electrode active materials having discharge capacities which are at least 20% higher than cobalt-based positive electrode active materials is actively being carried out. LiNiO2 is similar to LiCoO2 in having a layered structure and has an initial discharge capacity of 180-200 mAh/g, but due to being structurally unstable—transforming from a monoclinic structure to a hexagonal structure during charging and discharging—rapidly decreases in capacity when continuous charging and discharging are performed, has low thermal stability and poor cycle properties, and has the disadvantage in which quantitatively stoichiometric material synthesis is difficult. In order to overcome such limitations, there have been attempts to achieve structural stability by adding cobalt to LiNiO2, but here, the amount of the cobalt added must be at least 30 mol %, and thus there was a limitation of causing a relative decrease in capacity.
Due to such circumstances, the materials receiving the most attention as replacement positive electrode active materials for LiCoO2 are lithium nickel manganese cobalt oxides, that is, Li(NixCoyMnz)O2 (here, x, y, and z are each independently the atomic fractions of elements forming the oxide, where 0<x≤1, 0<y≤1, 0<z≤1, and 0<x+y+z≤1). This material is less expensive than LiCoO2 and has the advantage in being able to be used for high capacities and high voltages, but has the disadvantages of poor rate capability and poor high-temperature lifetime properties.
In order to overcome such limitations, a method for manufacturing a lithium transition metal oxide having a concentration gradient in metal composition has been proposed, wherein, after preparing a double layer by synthesizing a core material and then coating the outside thereof with a material of a different composition, the double layer is mixed with a lithium salt and heat treated to manufacture the lithium transition metal oxide. In this method, although the core and an external layer may be synthesized to have different metal compositions when synthesized, a continuous concentration gradient of the metal composition is insufficiently formed in the manufactured positive electrode active material. Thus, the improvement effect in terms of output properties is unsatisfactory, and there is a limitation of low reproducibility.